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Editors Selection IGR 18-1

Anatomical Structures: Pressures behind the lamina

Comment by Hanspeter Killer on:

67346 Intracranial pressure (ICP) and optic nerve subarachnoid space pressure (ONSP) correlation in the optic nerve chamber: the Beijing Intracranial and Intraocular Pressure (iCOP) study, Hou R; Zhang Z; Yang D et al., Brain Research, 2016; 1635: 201-208

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Recent literature has discussed the influence of a pressure gradient between the intraocular pressure (IOP) and the pressure in the subarachnoid space (SAS) of the optic nerve on glaucoma pathogenesis. Hou et al. Present an interesting paper on cerebrospinal fluid (CSF) pressure performed in eight healthy dogs. They report on a thorough animal model that allows for simultaneous measurement of cerebrospinal fluid (CSF) pressure in three CSF spaces: the lumbar site, intracranially and in the SAS of the optic nerve. The paper challenges the current view that CSF pressure is homogenous in all CSF spaces, e.g., lumbar CSF pressure, ventricular CSF pressure (ICP) and the optic nerve sheath pressure (ONSP). In addition to the CSF pressure, the pressure in the anterior chamber was measured as well.

The paper challenges the current view that CSF pressure is homogenous in all CSF spaces

At baseline, the authors found different pressures in the different CSF spaces with the highest pressure in the anterior chamber, followed by the ventricular pressure and the lumbar pressure. The lowest pressure was found within the optic nerve sheath (subarachnoid space). When CSF was shunted, the ICP gradually decreased together with the optic nerve sheath pressure in a linear fashion. When the CSF pressure was further decreased to what the authors call a critical breakpoint, ICP and optic nerve sheath pressure became uncoupled and the optic nerve sheath pressure remained constant despite further ICP lowering (ICP independent zone). This phenomenon was described as a communication arrest between the ICP and the optic nerve sheath pressure. This phenomenon was called optic nerve sheath compartmentation by other authors in previous work carried out with computed assisted cisternography.¹

The results of this experimental study sheds light on concepts such as the translaminar pressure gradient and the optic nerve sheath compartmentation syndrome. Both concepts are thought to be involved in the pathogenesis of normal-tension glaucoma. Applying the Bernoulli equation, the pressure differences found between the single CSF spaces can help to explain the development of CSF compartmentation.

This paper is indeed especially interesting for the study of optic nerve sheath compartmentation as it clearly contradicts the idea of a continuous CSF pressure and flow on an experimental base. The experiment also renders data concerning the concept of the translaminar pressure gradient as a component of glaucomatous optic nerve damage.

I would like to congratulate the authors for this carefully performed experiment and the clear conclusions they draw from their data.

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